Inconsistent query result caused by index

-- case 1

CREATE TABLE t1 (c1 TEXT) ENGINE=InnoDB;

INSERT INTO t1 (c1) VALUES ('a');

SELECT c1 FROM t1 WHERE CAST(c1 AS BINARY(7)) NOT IN (SELECT c1 FROM t1); -- {a}

-- case 2

CREATE TABLE t1 (c1 TEXT, UNIQUE (c1(1))) ENGINE=InnoDB;

INSERT INTO t1 (c1) VALUES ('a');

SELECT c1 FROM t1 WHERE CAST(c1 AS BINARY(7)) NOT IN (SELECT c1 FROM t1); -- actual: {}, expected: {a}